Characterization of oil fields often occurs through the analysis of rock samples removed from the earth during drilling operations. One method of analysis is pyrolysis, in which a rock sample is heated in an inert environment, causing free hydrocarbons and kerogen-based hydrocarbons to decompose. Analyzing the gases released from decomposed hydrocarbons may then help categorize the size and maturity of the oilfield.
At temperatures below 350° C., the gases are released from free hydrocarbons. The amount of free hydrocarbons in the rock sample is called the S1 parameter. The S1 parameter may be determined through pyrolysis heating programs, which may include heating a rock sample for a period of time at a temperature less than 350° C., followed by heating a rock sample for a period of time at a temperature of 300° C. or 350° C.
At temperatures below about 800° C., kerogen in the rock sample is cracked and converted into heavier hydrocarbons. The amount of kerogen-derived hydrocarbons is called the S2 parameter. The S1 and the S2 parameter may be used to help classify the rock and the hydrocarbons contained therein.
The density of oil is often compared to the density of water using the American Petroleum Institute (API) gravity index. A higher API gravity indicates a lower petroleum density. Crude oils with a high API gravity are often more profitable than similarly accessed crude oils with a low API gravity. API gravity of a rock sample is usually determined in a lab by processing the crude oil from a rock sample and comparing it to that of water.